1. Field of the Invention
The present invention relates to a magnetic random access memory (MRAM) device, and in particular, to a circuit that adjusts the pulse width of a write current through an MRAM device that uses magnetic memory cells storing data on the basis of the tunneling magnetoresistive effect.
2. Description of the Related Art
In recent years, a large number of memories have been proposed which store information on the basis of new principles. One of them is a non-volatile and high-speed MRAM device having magnetic memory cells arranged in a matrix and each composed of a magnetic tunnel junction (hereinafter referred to as an “MTJ”) element that stores “1”/“0” information using the tunneling magnetoresistive effect. Such an MRAM device is disclosed in, for example, Roy Scheuerlein et. al. “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”, ISSCC2000 Technical Digest pp. 128 to pp. 129.
The MTJ element has two magnetization layers. To program data in the MTJ element, a magnetic field in a predetermined direction is applied to the MTJ element to change the orientation of magnetization in one of the two magnetization layers.
A switching magnetic field required to program data in the MTJ element is dependent on temperature. The magnitude of the switching magnetic field decreases at high temperature. The dependence of the switching magnetic field on temperature is disclosed in, for example, M. Bhattacharyya, et. al., “Thermal Variations in Switching Fields for Sub-Micron MRAM Cells”, IEEE TRANSACTIONS ON MAGNETICS, VOL. 37, NO. 4, JULY 2001, pp. 1970–1972.
Further, the following point is disclosed in T. Honda, et. al., “MRAM-Writing Circuitry to Compensate for Thermal-Variation of Magnetization-Reversal Current”, 2002 Symposium on VLSI Circuits Digest of Technical Papers: the magnitude of a write magnetic field is desirably varied depending on temperature in order to prevent miswrites.
Furthermore, the following point is disclosed in M. Motoyoshi, et. al., “High-Performance MRAM Technology with an Improved Magnetic Tunnel Junction Material” 2002 Symposium on VLSI Technology Digest of Technical Papers: the switching magnetic field applied to the MTJ element for a write is dependent on the time for which the magnetic field is applied, and its magnitude increases inconsistently with decreasing pulse width of a pulse-like write current.
In connection with the dependence of the switching magnetic field on its applied time and temperature, even if the write current remains unchanged, writes can be achieved at a higher temperature even with a reduced pulse with of the pulse-like write current. Accordingly, miswrites are more likely to occur. If a circuit used to determine the pulse width of the write current is simply composed of CMOS inverter circuits, the CMOS inverter circuits operate at a reduced speed at a higher temperature and then the pulse width is increased. This increases the pulse width of the write current to make miswrites more likely to occur.
P. R. Gray and R. G. Mayor, in “Analysis and Design of Analog Integrated Circuits, Second Edition” John Wiley & Sons, Inc., 1984, p.289 to 296, disclose a circuit that adds an output from a first current source providing a current having a value increasing in proportion to temperature to an output from a second current source characterized by providing a current having a value decreasing in proportion to temperature, in order to provide a current source that is independent of temperature.
Further. U.S. Pat. No. 6,081,445 discloses a method of reproducibly rewriting the directions of magnetization in the MTJ element. With this method, data is written by applying a magnetic field Hy in a hard-axis direction to adjust the direction of magnetization in a magnetic domain at an end of a storage layer, and then applying a magnetic field Hx in an easy-axis direction.
When data is programmed in the MTJ element and if the write current remains unchanged, writes can be achieved at a higher temperature even with a reduced pulse with of the pulse-like write current. Accordingly, miswrites are more likely to occur.
If the circuit used to determine the pulse width of the write current is composed of CMOS inverter circuits, CMOS inverter circuits operate at a reduced speed at a higher temperature and then the pulse width is increased. This increases the pulse width of the write current, making miswrites more likely to occur.
As described above, if the circuit used to determine the pulse width of the write current is simply composed of CMOS inverter circuits, miswrites are more likely to occur at a higher temperature. Consequently, it has hitherto been desirable to allow the write current to be controlled to be independent of temperature or to have a desired temperature dependence. This prevents data miswrites over a wide temperature range to enable stable writes.